Question

Arrange the following compound in decreasing order of rate of electrophilic aromatic substitution reaction.

• A. (I) > (III) > (II) > (IV)
• B. (III) > (I) > (II) > (IV)
• C. (I) > (II) > (III) > (IV)
• D. (I) > (IV) > (II) > (III)

Answer 1

The correct option is A (I) > (III) > (II) > (IV)

The decreasing order of rate of electrophilic aromatic substitution reaction is $\left(I\right)>\left(III\right)>\left(II\right)>\left(IV\right)$.

Alkyl groups activate the benzene ring towards electrophilic aromatic substitution. They are ortho-para directing groups.

Compound (I) has the highest rate towards electrophilic aromatic substitution as it contains 3 methyl groups which are either ortho or para to incoming electrophile.

Compound (IV) has the lowest rate towards electrophilic aromatic substitution because it contains only one methyl substituent.

Compound (III) has a higher rate than compound (II) towards electrophilic aromatic substitution because in compound (III), the two methyl groups are either ortho or para to the incoming nucleophile whereas in the compound (II), one methyl group is ortho and another methyl group is para to the incoming electrophile.